Phase Separation and Crystallization in Glass

 

From the Mo-Sci Corporation Blog:

Posted by Krista Grayson 

While historically a source of problems for glass producers, the phenomenon of phase separation is now known to offer advantages in the production of certain materials such as glass ceramics and porous glasses. Whether desirable or undesirable, understanding and controlling phase separation during the glass manufacturing process is crucial. In this article, we explore the basics of phase separation and how it can be manipulated to create advanced materials for various applications.

What is phase separation?

In physics and chemistry, the word “phase” refers to a region of a material that is chemically uniform and physically distinct. Phase separation, which typically occurs in liquids, is where a homogeneous mixture separates into two or more of these phases. For example, a mixture of water and oil at room temperature will naturally “phase separate” into a distinct phase consisting of pure oil, and another consisting of pure water. We can say that such a mixture is “immiscible.”

The morphology of this phase separation can vary depending on the relative concentration of both components. If the mixture is predominantly water, the oil phase will take the form of distinct (or “discontinuous”) droplets dispersed throughout an interconnected (or “continuous”) water phase. If the mixture is predominantly oil, the opposite will take place. At roughly equal proportions of oil and water, each phase will tend to be continuous.

Phase Separation in Glass

Phase separation commonly occurs in glass melts. Borosilicate glass – which contains both silica and borate as network formers – is a well-studied example.1,2

Unlike our water/oil example, phases in glass melts are not necessarily chemically pure. Borosilicate glass, for example, will typically undergo phase separation into a “borate-rich” phase and a “silica-rich” phase, with both phases containing different proportions of each network former. In addition, the morphology of separated phases in glass can vary. While it is possible for droplet-like phases to form via classical nucleation and growth, spontaneous “spinodal” phase separation can result in the formation of intertwined tendril-like continuous phases.3

On the left, spinodal decomposition produces “tendrils” of different phases. On the right, nucleation produces droplets of the darker phase within the lighter phase. (Gebauer et al., 2014)4

This phase separation, which occurs at high temperatures in the molten glass, persists and “freezes in” when the glass is cooled into a solid. If both phases are vitrifiable, they may form glasses after cooling (this is called a glass-glass phase separation). However, if one phase is prone to crystallization, the mixture can cool into a glass-crystal phase-separated solid.5

Phase separation in glasses was long seen as undesirable – and for many applications, it still is.6 The existence of different phases modifies the physico-chemical properties of glass melts, making it difficult to mold and reduce the quality of the final glass.

The physics of phase separation in glass-forming materials is complex, and even today the specifics are subject to intense debate.7 However, glass manufacturers nonetheless determined ways of avoiding or minimizing phase separation during glass manufacturing.

Typically, this is achieved by tailoring the composition of glass melts, with phase separation only occurring for specific compositions. In a Na2O–B2O3–SiO2 glass system, for example, the following ternary phase diagram shows the immiscibility region in which phase separation will occur.

Simplified ternary phase diagram for the Na2O–B2O3–SiO2 system. (Bartl et al., 2001)8

Phase separation (and subsequent crystallization) can also be controlled by the addition of glass modifiers, and by varying heat treatment and cooling rates.9

Controlling and Exploiting Phase Separation in Glass

Note that within the immiscibility region in the diagram above, two common commercial glass compositions are labeled. Indeed, it’s now understood that phase separation ­offers advantages in certain applications. Today, heterogeneous phase-separated glasses cover a broad range of commercial applications, including Pyrex®, Vycor® opal glass, porous glass, and glass ceramics.

Glass-ceramics are a class of polycrystalline materials that share many properties with both glasses and ceramics, ideally providing the moldability of glasses with various special properties (such as high strength) of ceramics. Glass-ceramics are produced by the formation of crystal phases within an amorphous base glass (i.e., crystal-glass phase separation). Engineering glass-ceramics depends on controlling crystallization within the base material.10

Another application of controlled phase separation is in the production of porous glasses. Porous glasses are typically high-silica glasses that contain pores with a specific size distribution, ranging from angstrom to millimeter scales. Porous glasses are commonly produced from phase separation of alkali borosilicate glass, in which the mixture undergoes spinodal phase separation following heat treatment to yield two continuous phases.11 Following phase separation, the alkali-rich borate phase can be dissolved in acid and removed from the solid. This leaves a highly pure and porous silica glass “skeleton.”

Schematic showing the formation of porous glass from a phase-separated alkali (sodium) borosilicate mixture. (Hasanuzzaman et al 2016)11

Porous glass exhibits improved mechanical and thermal stability compared to ordinary bulk glass, making it a popular alternative to fused quartz which is comparatively difficult to form. Other applications make use of the pores themselves: such as filitering materials, catalyst supports, and targeted drug delivery.12–16 Mo-Sci is a world-leading provider of advanced glasses for healthcare, electronics and engineering applications. We offer a range of glass-ceramic seals and porous glass solutions, as well as providing custom solutions for virtually any glass application. Contact us for more information.

References and Further Reading

1. Charles, R. J. Phase Separation in Borosilicate Glasses. Journal of the American Ceramic Society 47, 559–563 (1964).
2. Möncke, D., Ehrt, D. & Kamitsos, E. Spectroscopic study of manganese-containing borate and borosilicate glasses: Cluster formation and phase separation. Physics and Chemistry of Glasses – European Journal of Glass Science and Technology Part B 54, 42–51 (2013).
3. Bergeron, C. G. & Risbud, S. H. Introduction to Phase Equilibria in Ceramics. (American Ceramic Society, 1984).
4. Gebauer, D., Kellermeier, M., Gale, J., Bergström, L. & Cölfen, H. Pre-nucleation clusters as solute precursors in crystallisation. Chemical Society reviews 43, 2348–2371 (2014).
5. Schuller, S. Phase separation in glass. (2018).
6. Morey, G. W. The Properties of Glass. (Books on Demand, 1954).
7. Da Vela, S. et al. Interplay between Glass Formation and Liquid–Liquid Phase Separation Revealed by the Scattering Invariant. J. Phys. Chem. Lett. 11, 7273–7278 (2020).
8. Bartl, M. H., Gatterer, K., Fritzer, H. P. & Arafa, S. Investigation of phase separation in Nd3+ doped ternary sodium borosilicate glasses by optical spectroscopy. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 57, 1991–1999 (2001).
9. Liu, S., Zhang, Y. & Yue, Y. Effect of cooling rate on crystallization in an aluminophosphosilicate melt. Physics and Chemistry of Glasses – European Journal of Glass Science and Technology Part B 52, (2011).
10. Control of nucleation in glass ceramics | Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences. https://royalsocietypublishing.org/doi/10.1098/rsta.2002.1152.
11. Hasanuzzaman, M., Rafferty, A., Sajjia, M. & Olabi, A.-G. Production and Treatment of Porous Glass Materials for Advanced Usage. in Reference Module in Materials Science and Materials Engineering (Elsevier, 2016). doi:10.1016/b978-0-12-803581-8.03999-0.
12. Hammel, J. J. & Allersma, T. United States Patent | Thermally stable and crush resistant microporous glass catalyst supports and methods of making. 341–341 (1975).
13. Jungbauer, A. Chromatographic media for bioseparation. Journal of Chromatography A 1065, 3–12 (2005).
14. Sotomayor, P. T. et al. Construction and evaluation of an optical pH sensor based on polyaniline-porous Vycor glass nanocomposite. in Sensors and Actuators, B: Chemical vol. 74 157–162 (2001).
15. Takahashi, T., Yanagimoto, Y., Matsuoka, T. & Kai, T. Hydrogenation activity of benzenes on nickel catalysts supported on porous glass prepared from borosilicate glass with small amounts of metal oxides. Microporous Materials 6, 189–194 (1996).
16. Using Porous Glass Microspheres for Targeted Drug Delivery Mo-Sci Corporation. https://mo-sci.com/porous-glass-microsphers-targeted-drug-delivery/.

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Controlled Pore Glass Manufacturing and Applications

 

Controlled Pore Glass Manufacturing and Applications

Posted Krista Grayson on Oct 29, 2020

Controlled pore glass (CPG) is a high-silica glass that contains pores with a specific size distribution. Porous glasses can be made into a wide range of geometric forms (such as frit, rods, plates, beads, and hollow spheres), and pore sizes can be precisely tuned from the range of angstroms to millimeters. Controlling pore size means that the physical and chemical reactivity of the glass with gases and liquids can be tailored to specific applications such as chromatography, sensing, and filtering.

In addition to this, porous glasses exhibit high mechanical strength, chemical durability, and thermal stability; which make them superior to other porous media (such as polymers and ceramics) for a variety of applications.1

This article covers how porous glass is made, how pore size can be controlled, and some of the varied applications of this unique material. 

Manufacturing Porous Glass

Porous glass can be manufactured via several different routes, each of which produces different characteristic pore structures. The most common methods involve phase separation or immiscibility of alkali borosilicate glass.

Producing controlled pore glass via the alkali borosilicate system

Alkali borosilicate glass systems consist of a silica glass-former with borate and alkali-oxide additives used to lower the melting temperature of the mixture and impart other properties. In other terms, alkali borosilicate systems are mixtures consisting of the chemical species SiO2, B2O3, and R2O; where R is sodium, potassium, or lithium.

Simplified ternary phase diagram for the Na2O–B2O3–SiO2 system. The “Vycor” region corresponds to the phase separable mixtures that can be used to manufacture porous glass. (Bartl et al., 2001)

Schematic showing the formation of porous glass from a phase separated alkali (sodium) borosilicate mixture. (Hasanuzzaman et al 2016)

When the constituents of this mixture are tuned to specific concentrations and heated, the entire mixture undergoes an amorphous phase separation: the mixture transforms into two distinct phases.

One of these phases is an alkali-rich borate phase and the other a silica-rich glassy phase. Crucially, the borate phase is soluble in acid, while the silica phase is not. This means that, following heat treatment, the borate phase can be leached out with a hot acid solution. What remains is a highly pure and porous silica glass skeleton with large surface area: in other words, porous glass.

Controlling pore size

Acid-leaching of a phase-separated mixture generally results in a very narrow pore size distribution, earning the name “controlled-pore glass” and lending the resulting glasses to applications such as adsorptive chromatography of biomolecules.2

The average pore diameter is a function of heat treatment temperature and time, as well as glass composition. Thus, controlling the heat treatment temperature or time (or both) can easily produce porous glasses with a range of pore sizes to suit different applications. Glasses formed via these methods generally have pore diameters in the region of 1 to 1000 nm.3,4

Formation of porous glass using alkali borate systems can also be achieved without inducing a high-temperature phase separation: directly etching the surface of the glass can result in the formation of small pores (1-2 nm) restricted to the surface of the glass.

Other manufacturing routes

Porous glass can also be manufactured by glass sintering or via sol-gel routes. Glass sintering is widely used to produce glass foams with pore diameters in the region of 400 m to 1 mm. In sol-gel processes, a solution of organic monomers (sol) is turned into a glass by removal of the liquid phase. Sol-gel processes have been used successfully to create a range of pore sizes for different applications5,6 and they are becoming more common methods.

Applications of Porous Glasses

Porous glass provides an alternative to fused quartz which is comparatively difficult to produce and form into different geometries. However, many emerging applications make use of the functionality offered by the pores themselves. The high surface area and tailorable pore size distribution of these glasses make porous silica a highly effective filtering material, capable of separating not only the basis of molecular size but also of molecule type.7 This, along with a wide range of possible geometries, has made them useful in biosciences and chemistry.1

For example:

• Enzyme immobilization and size exclusion chromatography techniques have been developed using porous glass; making use of its extreme chemical inertness, optical transparency, and small pore diameters.5,8,9
• Surface-functionalization of controlled-pore glass using polyaniline has been used to develop optical chemosensors.10
• Using additives to finely tune the size of pores can result in functional size-selective catalyst supports.11,12
• The role of porous glass in targeted drug delivery has been studied, using porous-wall hollow glass microspheres. The spheres provide a porous, inert shell for the introduction and release of drugs inside the body.13
• Porous glass is also being investigated as a bio-scaffold. These applications make use of the porosity, strength, corrosion-resistance, and biocompatibility of porous glass.14,15

All of these applications are made possible by the tunability of pore size, which enables specific physical properties to be imparted in the glass during the manufacturing process.

Mo-Sci produces high purity (> 98% SiO2 and < 2% B2O3) porous glass frit and spheres suitable for applications in industry and research. Contact us to speak with one of our experts about your project requirements.

References and Further Reading

1. Hasanuzzaman, M., Rafferty, A., Sajjia, M. & Olabi, A.-G. Production and Treatment of Porous Glass Materials for Advanced Usage. in Reference Module in Materials Science and Materials Engineering (Elsevier, 2016). doi:10.1016/b978-0-12-803581-8.03999-0
2. Elmer, T. H. Porous and Reconstructed Glasses. in Engineered Materials Handbook (1992).
3. Zhu, B. et al. Synthesis and Applications of Porous Glass. J. Shanghai Jiaotong Univ. 24, 681–698 (2019).
4. Enke, D., Janowski, F. & Schwieger, W. Porous glasses in the 21st century-a short review. Microporous Mesoporous Mater. 60, 19–30 (2003).
5. Lubda, D., Cabrera, K., Nakanishi, K. & Minakuchi, H. SOL-GEL PRODUCTS NEWS Monolithic HPLC Silica Columns. Journal of Sol-Gel Science and Technology 23, (2002).
6. Baino, F., Fiume, E., Miola, M. & Verné, E. Bioactive sol-gel glasses: Processing, properties, and applications. Int. J. Appl. Ceram. Technol. 15, 841–860 (2018).
7. Hammel, J. J. & Allersma, T. United States Patent | Thermally stable and crush resistant microporous glass catalyst supports and methods of making. 923, 341 (1975).
8. Du, W. F., Kuraoka, K., Akai, T. & Yazawa, T. Effect of additive ZrO2 on spinodal phase separation and pore distribution of borosilicate glasses. J. Phys. Chem. B 105, 11949–11954 (2001).
9. Jungbauer, A. Chromatographic media for bioseparation. Journal of Chromatography A 1065, 3–12 (2005).
10. Sotomayor, P. T. et al. Construction and evaluation of an optical pH sensor based on polyaniline-porous Vycor glass nanocomposite. in Sensors and Actuators, B: Chemical 74, 157–162 (2001).
11. Takahashi, T., Yanagimoto, Y., Matsuoka, T. & Kai, T. Hydrogenation activity of benzenes on nickel catalysts supported on porous glass prepared from borosilicate glass with small amounts of metal oxides. Microporous Mater. 6, 189–194 (1996).
12. Gronchi, P., Kaddouri, A., Centola, P. & Del Rosso, R. Synthesis of nickel supported catalysts for hydrogen production by sol-gel method. in Journal of Sol-Gel Science and Technology 26, 843–846 (Springer, 2003).
13. Using Porous Glass Microspheres for Targeted Drug Delivery Mo-Sci Corporation. Available at: https://mo-sci.com/porous-glass-microsphers-targeted-drug-delivery/. (Accessed: 2nd September 2020)
14. Rahaman, M. N. et al. Bioactive glass in tissue engineering. Acta Biomater. 7, 2355–2373 (2011).
15. Fu, Q., Saiz, E. & Tomsia, A. P. Bioinspired strong and highly porous glass scaffolds. Adv. Funct. Mater. 21, 1058–1063 (2011).